(a) Field of the Invention
This invention relates to improvements in the process for the bipolar refining of lead and, more particularly, to a method for improving the efficiency of the process.
(b) Description of the Prior Art
In bipolar refining of lead, a number of lead bullion electrodes are immersed in an electrolytic cell containing a lead fluosilicate-fluosilicic acid electrolyte. Only the first and last electrodes in the cell are connected to a source of direct electrical current, the remainder of the electrodes being left unconnected to the current source. The current causes lead to dissolve from the lead bullion electrodes leaving a layer of slimes containing impurities such as, for example, bismuth, arsenic and antimony, adhering to the anodic side of the electrodes, and causes dissolved lead to deposit as refined lead on the cathodic side of the electrodes. Upon completion of the refining cycle, electrodes are removed from the cell and slimes and refined lead are stripped from the electrodes. The efficiency of this process is high and is much improved over that of the conventional Betts Process. Supply of electrical power to cell and electrodes is vastly simplified, current densities can be much higher and mechanization is possible to a much greater degree than with the Betts Process. The process for the bipolar refining of lead is described in detail in our U.S. Pat. No. 4,177,117, which issued Dec. 4, 1979.
Although the bipolar refining process has many advantages over the Betts Process, control of the process has been found to be difficult when the process is operated at high current densities. Maintaining the desired low impurity content of the refined lead becomes more difficult with increasing current densities, in spite of operating at the optimum current-voltage relationship to prevent the anode overvoltage from exceeding the voltage at which impurities dissolve from the lead bullion. In addition, at high current densities the layer of slimes which remains adhering to the anodic side of the bipolar electrodes becomes less stable. Detachment of the slimes from the anodic side of the bipolar electrodes results in an increasing amount of slimes in the electrolyte and of impurities in the refined lead. The control of electrical shorting in the cell becomes more difficult, particularly because higher than average current densities at the edges of the electrodes tend to result in undesirable nodular and dendritic growths. Because it is also desirable to maintain close spacings between the electrodes and the cell walls, such growths may also occur across the gap between electrodes and cell walls. Electrical shorting also occurs at a higher incidence at the end electrodes than at the other electrodes in the cell. Electrical shorting can only be partly controlled by monitoring the cathode polarization voltage and maintaining optimum amounts of addition agents in the electrolyte. The lead deposited at high current densities tends to become coarser, less dense and more brittle which results in difficulties when the refined lead is to be stripped from the electrodes.